The invention relates to static power systems in general, and more particularly to uninterruptible power systems (UPS) such as used on aircraft, for computer systems, or wherever a source of proper nature, voltage and/or frequency might become disrupted and when continuity of source is essential.
The function of a UPS system is to supply electrical power to certain critical loads when the main source of electrical power fails. Therefore, UPS systems usually spend most of their useful life standing by, waiting to supply power on an instant's notice. Hence, power consumption in standby is an important consideration. For many applications the continuity of power is of vital importance. Disruptions exceeding a fraction of an AC cycle cannot be tolerated. In addition, in many cases the quality of the output voltage provided by the UPS system is required to be comparable to that of the primary power system. Only minor impairments in voltage quality are tolerated immediately after load transfer to UPS supply. Moreover, under steady state operation it is required to filter out any voltage distortions caused by the UPS power circuit, and to provide a low output impedance path for the harmonic components of the load current such as drawn by rectifiers which is rich in harmonics.
Due to the usually low source impedance provided by the primary electrical system, the transfer switch contacts connecting the load to the electrical system are subject to large fault currents. An important consideration is to design these contacts with a high fault current rating without, however, adversely affecting the speed of the transfer or the cost of this portion of the UPS system.
The invention pertains to load transfer from the primary AC power system to operation under the standby source of a UPS system.
Load transfer is effected with a transfer switch apparatus for coupling the load to either the primary AC power supply or the UPS system. The latter consists of a DC to AC inverter system fed from a standby DC source, typically a battery. A UPS system generally also includes a harmonic filter which needs to be connected as a bypass to the primary AC power supply line, once the harmonic generating inverter is in operation with the load.
The object of the present invention is to provide minimal power consumption in the UPS system when in the standby mode.
Another purpose of the invention is to allow a fast load transfer in a UPS system which is compatible with minimal interruption of the load power supply.
The invention further affords in a UPS system, the generation immediately upon transfer to the standby source, of a good quality inverter output voltage with the standby source in operation.